Using viscous couplings in the driveline of a motor vehicle with a permanently driven first axle and a second axle which is driven when required, wherein the parts of the coupling are connected to an input end and an output end in a driveline for a second axle is known and constitutes the most frequent application. In particular, it is also known to combine the viscous coupling with a freewheeling assembly which has a locking effect if the input end leads relative to the output end and which has an unlocking effect if the output end leads relative to the input end; this applies to forward driving of the vehicle. The disadvantage is that when the vehicle reverses, i.e. when the direction of rotation is reversed, the freewheeling unit unlocks the viscous coupling and the second axle cannot be driven; even if there is no grip at all at the first axle, the second axle cannot be connected. To overcome this disadvantage, it was necessary to provide an additional switching device by means of which the freewheeling assembly is bridged again during reversing.
The present invention is embodied in and carried out by a viscous coupling having two parts for the transmission of torque between a first and a second of said two parts, causes by a speed differential between said first and said second of said two parts, which two parts are supported inside one another, and which are rotatable relative to one another around a common longitudinal axis, and which form an annular chamber which is filled with a highly viscous fluid and in which first coupling plates are non-rotatably connected to the first one of the rotatable parts, e.g., the hub, and second coupling plates are non-rotatably connected to the second one of the rotatable parts, e.g., the housing, are arranged alternately in the longitudinal direction, and which, in the general rotational direction, i.e., if used in a vehicle during forward driving, carries out the functions of a freely switchable viscous coupling and which is lockable in the opposite rotational direction, i.e., when reversing if having slip at the first axle, without additional means.
The objective is achieved by providing a differential speed-dependent freewheeling assembly which, if torque flows in a first direction at a positive speed differential between the first and the second of said rotatable parts, has a locking effect in both directions of relative rotation of its rings at least above a relative rotational acceleration and which, if torque flows in a second direction at a negative speed differential between the first and the second of said rotatable parts, has an unlocking effect in both directions of rotation of its rings, and which freewheeling assembly is effective between the one of the parts which are rotatable relative to one another and a plate carrier which carries at least part of the coupling plates of this particular one of the parts which are rotatable relative to one another.
If only part of the coupling plates can be coupled to the respective rotatable part by means of the plate carrier, whereas the remaining coupling plates associated with that part are permanently and non-rotatably connected to same, the coupling, in the differential speed range starting with zero, initially comprises the characteristic curve of a conventional viscous coupling. When the second part of the coupling plates is connected by means of the plate carrier as a function of a minimum differential speed, it is possible, from a certain differential speed onwards, to change over to the characteristic curve of a correspondingly larger viscous coupling.
If the plate carrier is connected as a function of a certain relative rotational acceleration, the change-over from one characteristic curve to the other characteristic curve takes place independently of the absolute differential speed as a function of the rotational acceleration. If all coupling plates of the one of the parts which are rotatable relative to one another can be coupled to or uncoupled from that particular part by means of the plate carrier, there is no presence of the characteristics of the conventional viscous coupling, and the coupling as a whole can be connected either as a function of the differential speed or as a function of the rotational acceleration. The advantageous effect of this feature is that, during maneuvering, the inventive viscous coupling is not locked or only slightly locked and that the second driving axle is effectively coupled only if considerable slip occurs at the first driving axle. As a result, the driveline distortions to be avoided during maneuvering are not built up. This applies to forward driving and reversing.
When the torque flow is reversed, i.e. in the case of a negative speed differential (front axle being rolled over by the rear axle), the freewheeling unit unlocks without fail. This driving condition is realistically conceivable during forward driving only, i.e. when the first driving axle is blocked.
According to a special embodiment, the freewheeling assembly comprises locking members which are held in a cage which is brakeable (retainable) relative to the one of the parts rotatable relative to one another and which, when braked (retained) causes the freewheeling unit to be blocked. In this context, it is proposed in particular that the cage is connected to a brake plate which is positioned inside the sealed annular chamber and which adjoins a radial wall of the other one of the parts which are rotatable relative to one another. In the case of a slowly increasing speed differential between the coupling plates of the first part and the coupling plates connected to the plate carrier, the latter are driven at identical speeds. In the case of a suddenly increasing and/or upper speed differential, the plate of the cage with the adjoining wall of the other part stays behind relative to the one part, so that the freewheeling unit is blocked and the coupling carrier is coupled to the second part. In this way, the coupling becomes effective and changes to different characteristics.
The locking members of the freewheeling unit are positioned in a central position between ramp faces at the one of the two rings, whereas they co-operate with a centric annular face at the other one of the two rings. There are provided spring means which return the locking members in the circumferential direction into the non-locking central position.